1. Field of the Invention
This invention relates to a method of logging a subsurface formation penetrated by a wellbore to determine oil saturation. In particular, this invention relates to a method by which clay and shale damage is minimized in a log-inject-log procedure of determining oil saturation.
2. Setting of the Invention
In the planning or engineering of the production of oil from an underground formation, it is important to know the amount of oil in place. Oil in underground reservoirs is contained in the pores of the rock. However, these pores also contain, in addition to oil, water and frequently gas.
There are several ways to determine the percent of rock volume which is porous space. One such way is to cut a core, which typically is a cylindrical piece of the rock which is 2 to 3 inches in diameter and usually 20 to 30 ft. in length. After recovery of the core to the surface its porosity is determined. However, knowledge of the porosity of the rock does not give a complete knowledge of the amount of oil in the rock. One must still determine what part of the core pore space is filled with water, what part with oil and what part with gas. One can measure the quantities of gas, oil and water in the core that has been brought to the surface and determine the water, oil and gas saturations in the core. However, experience shows that the fluid content of the core at the surface is seldom the same as was the fluid content of the core in its natural condition in the reservoir.
Therefore, the formation preferably be analyzed in situ for oil saturation. However, the only logging device available that is described as having the capability to measure formation oil saturation directly is a carbon oxygen log which does not attain sufficient accuracy for measurement of oil saturation. Formation oil saturation means that percent of the fluid in the pore space of the reservoir rock, which is oil. There are devices, however, which can provide a measure of the water saturation. If a gas saturation exists, it can be measured by an independent method such as described in U.S. Pat. No. 3,282,095. Oil saturation can then be computed as the percent of pore space of the rock that is not filled by water or gas.
Pulsed neutron logs used in a log-inject-log sequence can be used to determine water saturations in wells which are cased or that are completed open hole. Usual procedure is to log a well in its true or native state and again after injecting a fresh water having a thermal neutron capture cross section nearly equal to that of formation hydrocarbons. This is called a log-injectlog procedure. An equation used to compute the formation oil saturation from this log-inject-log procedure assuming that no gas saturation exists, is EQU So=1-[(.SIGMA..sub.t -.SIGMA..sub.ff)/.phi.(.SIGMA..sub.w -.SIGMA..sub.hc)]
wherein
.SIGMA..sub.t =thermal neutron capture cross section of the formation as measured by the pulsed neutron log prior to injecting the freshwater. PA1 .SIGMA..sub.w =thermal neutron capture cross section of the formation water and can be predicted from chemical analysis of the formation water. PA1 .SIGMA..sub.ff =the thermal neutron capture cross section of the formation as measured by the pulsed neutron log after injecting fresh water to miscibly displace all of the formation water plus some of the formation oil. The thermal neutron capture cross section of the fresh water is adjected to be approximately equal to that of the formation hydrocarbon. PA1 .SIGMA..sub.hc =thermal neutron capture cross section of the formation hydrocarbons and can be estimated from compositional analysis of the oil at reservoir conditions. PA1 .phi.=the porosity, fraction of bulk volumes which is determined from core data and/or porosity log.
In such a logging technique where fresh water must be injected so as to have a fluid in the pore space with a proper thermal neutron capture cross section, several problems can occur with the clay and shale usually present in the formation. The clays in these formations are sometimes peptized or dispersed by the fresh water thereby causing permanent impairment of the formation. Further, shale may be caused to slough into the hole which also impairs the flow in and out of the formation. In such impairments, fluids will not flow as readily into or out of the formation as it would prior to injection of the fresh water.
3. Relevant Publication
U.S. Pat. No. 3,783,683, Robert P. Murphy et al., issued on Jan. 8, 1974, and entitled "Minimizing Clay Damage in a Log-Inject-Log Procedure" discloses a procedure by which the fresh water is treated to prevent clay impairment while maintaining a neutron capture cross section of the solution similar to that of fresh water which it replaces. The method includes the use of a divalent metallic salt solution whose cation has a thermal neutron capture cross section less than 0.6.times.10.sup.-24 cm.sup.2 and whose anions has no constituents having a thermal neutron capture cross section more than 0.6.times.10.sup.-24 cm.sup.2, the salt generating essentially a neutral pH upon dissolving in water and being at least 3000 ppm in the solution. Murphy preferred magnesium sulfate as the divalent metallic salt.
U.S. Pat. No. 4,142,595, Duane B. Anderson and Carl D. Edwards, issued Mar. 6, 1979 and entitled "Shale Stabilizing Drilling Fluid" discloses a drilling fluid which includes a salt having cation of either potassium or ammonium and having a concentration of 100,000 ppm in addition to flax seed gum and a non clay viscosifier. The preferred salts of Anderson include potassium chloride and ammonium sulfate. The drilling fluid of Anderson would not be suitable for injection into a formation as required in this invention.